1. Field of the Invention
This invention relates to semiconductor materials having enhanced thermoelectric properties and preparation of such materials.
2. Related Art
Thermoelectric generators convert heat energy directly into electrical energy without moving parts. They are reliable, operate unattended in hostile environments and are also environmentally friendly. The basic theory and operation of thermoelectric devices has been developed for many years. Such devices may be used for heating, cooling, temperature control, power generation and temperature sensing. Modern thermoelectric coolers typically include an array of thermocouples which operate by using the Peltier effect.
Thermoelectric devices are coolers, heat pumps, and power generators which follow the laws of thermodynamics in the same manner as mechanical heat pumps, refrigerators, or any other device used to transfer heat energy. The principal difference is that thermoelectric devices function with solid state electrical components (thermocouples) as compared to more traditional mechanical/fluid heating and cooling components. The efficiency of a thermoelectric device is generally limited to its associated Carnot cycle efficiency reduced by a factor which is dependent upon the thermoelectric figure of merit (ZT) of the materials used in fabrication of the thermoelectric device.
The dimensionless figure of merit ZT represents the coupling between electrical and thermal effects in a material and is defined as:
 ZT=S2σT/κ  (1)
where S, σ, κ, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively. The basic thermoelectric effects are the Seebeck and Peltier effects. The Seebeck effect is the phenomenon underlying the conversion of heat energy into electrical power and is used in thermoelectric power generation. The complementary effect, the Peltier effect, is the phenomenon used in thermoelectric refrigeration and is related to heat absorption accompanying the passage of current through the junction of two dissimilar materials.
ZT may also be stated by the equation:                     ZT        =                                            s              2                        ⁢            T                                ρ            ⁢                                                   ⁢            κ                                              (        2        )            
ρ=electrical resistivity
σ=electrical conductivity       electrical  conductivity    =                    1                  electrical  resistivity                    ⁢                           ⁢              or            ⁢                           ⁢      σ        =          1      ρ      
Thermoelectric materials such as alloys of Bi2Te3, PbTe and BiSb were developed thirty to forty years ago. Semiconductor alloys such as SiGe have also been used in the fabrication of thermoelectric devices. Commercially available thermoelectric materials are somewhat expensive. In addition, they are generally limited to use in a temperature range between 200 K and 1300 K with a maximum ZT value of approximately one. The efficiency of the thermoelectric devices using these materials remains relatively low at approximately five to eight percent (5-8%) energy conversion efficiency. For the temperature range of 200 to 300 K, maximum ZT of current state of the art thermoelectric materials remains limited to values of approximately 1, except for Te—Ag—Ge—Sb alloys (TAGS) which may achieve a ZT of 1.2 in a very narrow temperature range. Thermoelectric materials such as Si80Ge20 alloys used in thermoelectric generators to power spacecrafts for deep space missions have a ZT approximately equal to 0.7 from 500 to 1300 K.
However, for many applications with heat source temperature ranges between 100 C and about 350 C, there exists a gap between the low temperature state-of-the-art thermoelectric materials (Bi2Te3-based alloys) and the intermediate temperature materials (PbTe-based alloys) and TAGS (Te—Ag—Ge—Sb). Consequently, the applications of current thermoelectric materials are limited because of the relatively low efficiency of the thermoelectric materials as well as their relatively high cost.
Therefore, what is needed are more efficient new thermoelectric materials. In addition, what is needed are inexpensive thermoelectric materials. What is further needed are new thermoelectric materials with an expanded range of applications.
Whatever the merits of the prior techniques and methods, they do not achieve the benefits of the present invention.